1. Field of the Invention
The present invention relates generally to a planetary gear device, and more particularly to improvements in a planetary gear device having composite planetary gears each of which has a large-diameter pinion and a small-diameter pinion that are integrally formed with an axial spacing therebetween.
2. Discussion of the Related Art
A planetary gear device is widely used in a power transmitting system or assembly such as a transmission or speed reducing device for a motor vehicle. In one known type of such planetary gear device, a plurality of composite planetary gears are disposed around the axis of a carrier. Each of the composite planetary gears has a large-diameter pinion and a small-diameter pinion which are formed integrally with each other and arranged in the axial direction such that the teeth of the two pinions are spaced apart from each other in the axial direction. An example of this type of planetary gear device is disclosed in EP-A-0 63 895, which is adapted to be used in a power transmitting system of an electric car or vehicle. The use of two such planetary gear devices (each having composite planetary gears) disposed in series connection with each other makes it possible to provide a compact speed reducing system which has a relatively high speed ratio.
The present inventors previously developed a power transmitting system for an electric car, which incorporates such planetary gear device, as shown in FIG. 1. The developed power transmitting system is not publicly known at the time the present invention was made. The planetary gear device, which is indicated generally at 10 in FIG. 1, includes a sun gear 14, a carrier 16, three composite planetary gears 20 disposed on the carrier 16, and a ring gear 24. Each of the composite planetary gears 20 has a large-diameter pinion 26 and a small-diameter pinion 28 which are formed integrally with each other. Three planetary gears 20 are supported by the carrier 16 such that the three planetary gears 20 are arranged along the circumference of the ring gear 24 which has a center at the axis of the carrier 16, that is, at the center of the planetary gear device 10. The three planetary gears 20 are equally spaced apart from each other in the circumferential direction of the ring gear 24. The large-diameter pinion 26 of each composite planetary gear 20 meshes with a first gear in the form of the sun gear 14, while the small-diameter pinion 26 meshes with a second gear in the form of the ring gear 24. The power transmitting system has a hollow motor shaft 12, a rotary motion of which is transmitted to the sun gear 14. With the ring gear 24 functioning as a reaction element, the rotary motion of the sun gear 14 is transmitted to a differential gear device 36 through the carrier 16, such that the speed of the rotary motion of the carrier 16 is reduced at a predetermined ratio with respect to that of the sun gear 14. The rotary motion of the differential gear device 36 is transferred to transmission shafts 56, 58 which are connected to respective right and left wheels of the vehicle.
For reducing an operating noise of such planetary gear device, each of the gears used therein is generally a helical gear whose teeth are twisted obliquely to the axis or have a helix angle. The specifications such as the helix angle of the large-diameter and small-diameter pinions are determined independently of each other, depending upon the required mechanical strength and tooth contact ratio. Where the teeth of the two pinions have the same direction of helix, thrust forces act on the large- and small-diameter pinions in the opposite directions, due to the helix or twisting of the teeth. However, since the specifications of the two pinions are determined independently of each other, the two thrust forces do not completely offset each other, and the composite planetary gear as a whole is forced by the thrust forces in one axial direction. Therefore, the carrier supporting the composite planetary gears is required to have a comparatively high mechanical strength. Further, deflection or deformation of the carrier due to the thrust forces result in a loss of parallelism of the axes of the composite planetary gears and the first and second gears (sun gear and ring gear), which prevents smooth or adequate tooth engagement of these gears, leading to increased noise of the gears and reduced power transmitting efficiency of the planetary gear device.
With the thrust forces acting on the pinions of the composite planetary gears in one axial direction, the transmission of the rotary motion between the composite planetary gears and the carrier takes place while the gears are held in abutting contact with the carrier, namely, while the composite or overall thrust force is received by the carrier. Where the large- and small-diameter pinions have different lead amounts, the composite planetary gears cannot be moved independently of each other. In other words, all the composite planetary gears are synchronously moved together in the axial direction by the composite force. On the other hand, the carrier may have local dimensional errors, which may cause different amounts of axial movements of the individual composite planetary gears until the gears abut on the carrier. In this case, the thrust forces of all the composite planetary gears are not received by the carrier, and one or two of the gears is/are spaced apart from the carrier in the axial direction, whereby the thrust force of this gear or thrust forces of these gears is/are received by the mating first and second gears (sun gear and ring gear). Accordingly, the synchronous axial movements of the composite planetary gears may lead to deterioration of the tooth meshing or engaging condition of the gears, giving adverse influences on the gear operating noise, power transmitting efficiency and strength of the planetary gear device. Similar problems are encountered in the event that the axial movements of some of the composite planetary gears are more or less delayed, or in the event that some of the composite planetary gears are not able to move axially due to sticking, for example.
The planetary gear device 10 of FIG. 1 will be described in detail, by further referring to FIGS. 3 and 4. Where teeth 70, 72 of the large-diameter and small-diameter pinions 26, 28 of each composite planetary gear 20 have a left-hand helix, counterclockwise rotation of the sun gear 14 as seen in the right direction (from the left toward the right) in FIGS. 3 and 4 will cause a thrust force F.sub.SS to act on the large-diameter pinion 26 in the right direction as seen in FIG. 4, due to the helix of the teeth 70, while at the same time cause a thrust force F.sub.RS to act on the small-diameter pinion 28 in the left direction as also seen in FIG. 4, due to the helix of the teeth 72, as indicated in FIG. 3. The thrust force F.sub.SS is represented by F.sub.S .multidot.tan .beta..sub.S, where "F.sub.S " represents a force which is transferred from the sun gear 14 to the teeth 70 of the pinion 26 in the circumferential direction of the pinion 26, and ".beta..sub.S " represents a helix angle of the teeth 70. The thrust force F.sub.RS is represented by F.sub.R .multidot.tan .beta..sub.R, where "F.sub.R " represents a force which is transferred from the ring gear 24 to the teeth 72 in the circumferential direction of the pinion 28, and .beta..sub.R " represents a helix angle of the teeth 72. Since the helix angles .beta..sub.S and .beta..sub.R are determined differently independently of each other, the thrust force F.sub.SS =F.sub.S .multidot.tan .beta..sub.S and the thrust force F.sub.RS =F.sub.R .multidot.tan .beta..sub.R do not completely offset each other. As a result, the composite planetary gear 20 as a whole is forced by the composite or overall thrust force in one axial direction, and the composite or resultant thrust force is received by the carrier 16 through a thrust bearing 46 or 48.
Each composite planetary gear 20 is rotatably mounted on a pinion shaft 18, with a predetermined amount (e.g., about 1 mm or smaller) of axial clearance or play with respect to the thrust bearings 46, 48, for accommodating or absorbing dimensional errors of the various components of the planetary gear device 10. Upon application of a torque to the planetary gear device 10 during power transmission through the power transmitting system, the composite planetary gear 20 is axially moved to an appropriate one of its opposite axial ends by the above-indicated composite thrust force. Since the specifications of the large- and small-diameter pinions 26, 28 are determined independently of each other, an axial movement of one of the composite planetary gears 20 will cause relative rotation of the sun gear 14 and the ring gear 24. Consequently, unless all of the composite planetary gears 20 are synchronously moved in the axial direction by the same distance, the gears 20 are prevented from smoothly meshing the sun gear 14 and ring gear 24. In other words, non-synchronous axial movements of the gears 20 will cause undesirable contacts of the teeth at the back faces, or sticking of the teeth, whereby the operating noise of the gears 20 is increased, and the power transmitting efficiency and strength of the planetary gear device 10 are adversely influenced. In this respect, it is difficult manufacture the planetary gear device 10 such that all the composite planetary gears 20 have the same amount of axial clearance or play. This means that after one of the composite planetary gears 20 is brought into abutting contact with the carrier 16 through the thrust bearing 46, 48, the other composite planetary gears 20 are no longer axially movable toward the thrust bearing 46, 48, and the thrust forces of these gears 20 are not received by the carrier 16 and should be received by the mating sun and ring gears 14, 24, whereby the above-indicated problems are encountered.